Syp is a mammalian SH2-containing phosphotyrosine phosphatase (PTP) that was recently discovered during my post-doctoral studies, and is homologous to the Drosophila corkscrew gene product. Interestingly, this phosphatase appears to be a common target of receptor and cytoplasmic protein tyrosine kinases (PTKs), since Syp is able to physically bind to and is tyrosine phosphorylated by a number of ligand-activated growth factor receptors. Syp is constitutively phosphorylated on tyrosine in v-Src or Bcr-Abl transformed cells and forms a stable complex with Bcr-Abl and Grb2 in Bcr- Abl positive cells, suggesting a putative role of the PTP in neoplastic diseases. The PTP might also participate in insulin signaling since it binds to phosphorylated insulin receptor substrate 1 (IRS1) in adipocytes treated with insulin. While these biochemical evidences suggest that Syp is an important signaling component in normal and cancer cells, little is known about its biological significance. The objective of this project is therefore to uncover the Syp functions by a gene targeting approach. A Syp null mutation has been created in mouse embryonic stem (ES) cells and germ-line transmission of the mutant allele has been achieved from three independent ES cell clones. Preliminary results indicated that the homozygous Syp-/mutants die around day 8.5 of gestation. Our specific aims are: a). to analyze the phenotype of the Syp-/- mutant embryos, by focusing on the mesodermal development and patterning; b). to establish the Syp-deficient fibroblast cell lines from the knockout mice and to determine their signaling defects in response to mitogenic stimuli; c). to confirm the defective phenotype of the Syp- cell by rescue with the wild type Syp cDNA; d). to dissect the structure and functions of the PTP by transfecting the Syp- cells with Syp cDNAs mutated in its regulatory or catalytic domains and evaluating the ability of these mutants to rescue the Syp- phenotype; e). to examine the interaction of Syp with other signaling molecules in vivo by crossing the Syp knockout with various mutant mice. This study should provide fundamental insight into the molecular mechanism by which Syp acts in the control of signal transduction that regulates cell growth and differentiation. It will help us to understand the origin of cancer cells and to design new types of anti-cancer drugs.